Multi-stage turbocharger system

ABSTRACT

A turbocharger system comprises a first relatively small high-pressure (HP) turbocharger ( 1 ) and a second relatively large low pressure (LP) turbocharger ( 2 ). The turbine ( 6 ) of the LP turbocharger ( 2 ) is connected in series downstream of the turbine ( 4 ) of the HP turbocharger ( 1 ) in a first exhaust gas passage ( 11 ). An exhaust bypass flow passage ( 12 ) provides a bypass flow path around the HP turbine ( 4 ). A rotary valve ( 8 ) is located at a junction of the bypass flow passage ( 12 ) and a first exhaust gas flow passage ( 11 ). The rotary valve ( 8 ) comprises a valve rotor ( 19 ) which is rotatable to selectively permit or block flow to the LP turbine ( 6 ) from either the first exhaust gas passage ( 11 ) or the bypass gas passage ( 12 ).

The present invention relates to a multi-stage turbocharger system.Particularly, but not exclusively, the present invention relates to atwo stage turbocharger system.

Turbochargers are well known devices for supplying air to the intake ofan internal combustion engine at pressures above atmospheric pressure(boost pressures). A conventional turbocharger essentially comprises anexhaust gas driven turbine wheel mounted on a rotatable shaft within aturbine housing connected downstream of an engine outlet manifold.Rotation of the turbine wheel rotates a compressor wheel mounted on theother end of the shaft within a compressor housing. The compressor wheeldelivers compressed air to the engine intake manifold. The turbochargershaft is conventionally supported by journal and thrust bearings,including appropriate lubricating systems, located within a centralbearing housing connected between the turbine and compressor wheelhousings.

In known turbochargers, the turbine stage comprises a turbine chamberwithin which the turbine wheel is mounted; an annular inlet passagewaydefined between facing radial walls arranged around the turbine chamber;an inlet arranged around the inlet passageway; and an outlet passagewayextending from the turbine chamber. The passageways and chamberscommunicate such that pressurised exhaust gas admitted to the inletchamber flows through the inlet passageway to the outlet passageway viathe turbine and rotates the turbine wheel. It is also known to improveturbine performance by providing vanes, referred to as nozzle vanes, inthe inlet passageway so as to deflect gas flowing through the inletpassageway towards the direction of rotation of the turbine wheel.

Another known approach to improving turbocharging efficiency for anengine with a wide speed/load range is to provide a sequential two stageturbocharging system, comprising one relatively small high pressureturbocharger and another relatively large low pressure turbocharger. Theturbochargers are arranged in series so that exhaust from the engineflows first through the smaller turbine of the high pressureturbocharger and then through the larger turbine of the low pressureturbocharger. A valve controlled bypass passage is provided for allowingexhaust gas to bypass the high pressure turbine at high engine speedsand/or loads. Similarly, the compressors of the two turbochargers arealso arranged in series, with air flowing first through the relativelylarge compressor of the low pressure turbocharger and then through therelatively small compressor of the high pressure turbocharger. Again, avalve controlled bypass is provided to allow the inlet air to bypass thecompressor of the high pressure turbocharger at high engine speedsand/or loads.

It is an object of embodiments of the present invention to provide analternative or improved multi-stage turbocharger system.

According to the present invention there is provided a turbochargersystem comprising a first relatively small turbocharger; a secondrelatively large turbocharger; the first turbocharger comprising a firstexhaust turbine situated in a first exhaust passage; the secondturbocharger including a second exhaust turbine situated in said firstexhaust passage downstream of said first turbine; an exhaust bypass flowpassage communicating with the first exhaust flow passage upstream anddownstream of the first turbine; and a rotary valve located at ajunction of the bypass flow passage and the first exhaust gas flowpassage, the rotary valve comprising a valve rotor which is rotatableabout a valve axis to selectively permit or block flow to the secondturbine inlet from the first exhaust gas passage and/or bypass gaspassage.

In preferred embodiments the valve rotor rotates within a valve chamberwhich has a first port communicating with the first exhaust gas passage,a second port communicating with the bypass exhaust gas passage, and athird port communicating with the second turbine.

The valve rotor may be rotatable about said axis into a position inwhich each of the three ports is at least substantially unobstructed.This allows exhaust gas flow to bypass the first turbine. The valverotor may be rotatable about said axis into one of a plurality ofpositions in which the first port and third port are unobstructed andthe second port is at least a partially obstructed to modulate theamount of bypass flow.

The valve rotor may be rotatable into a position in which the secondport is fully obstructed. This may direct all exhaust gas flow throughthe first turbine.

The valve rotor may be rotatable about said axis into a position inwhich the second port is at least substantially obstructed and eitherthe third port or the first port is at least partially obstructed. Inthis position, and with fuel supplied to the engine, the exhaust gastemperature can be raised. The valve rotor may be rotatable about saidaxis into one of a plurality of positions in which the second port is atleast partially obstructed and either the third port or the first portis at least partially obstructed to modulate the exhaust heating effect.

In preferred embodiments, the exhaust heating is achieved by partiallyobstructing the third port rather than the first port. The first portmay be unobstructed. The second port may be fully obstructed.

The valve rotor may be rotatable into a position in which the first portis at least substantially obstructed and either the third port or thesecond port is at least partially obstructed. Thus may provide enginebraking. The valve rotor may be rotatable about said axis into one of aplurality of positions in which the first port is at least substantiallyobstructed and either the third port or the second port is at leastpartially obstructed to modulate the braking effect. The first port maybe fully obstructed. Preferably the second port is unobstructed and thethird port is at least partially obstructed.

The junction may be downstream or upstream of said first turbine.

In some embodiments the junction is downstream of the first turbine, thefirst port is an inlet port communicating with the first turbine, thesecond port is an inlet port communicating with the bypass exhaust gaspassage, and the third port is an outlet port communicating with thesecond turbine.

In some embodiments the junction is upstream of the first turbine, thefirst port is an inlet port communicating with the first exhaust gaspassage, the second port is an outlet port communicating with the bypassexhaust gas passage, and the third port is an outlet port communicatingwith the second turbine via the first turbine. In such embodiments thevalve rotor may be rotatable into at least one position in which thesecond port is at least substantially obstructed and the first port isat least partially obstructed. This will provide exhaust heating in anengine fired mode. Preferably the second port is fully obstructed andthe third port is unobstructed. The valve rotor may be rotatable into atleast one position in which the third port is at least substantiallyobstructed and the first port is at least partially obstructed. Thiswill provide engine braking. Preferably the valve rotor is rotatableinto a position in which the third port is fully obstructed and thesecond port is unobstructed.

The valve may comprise a barrel valve body rotatably housed within avalve chamber defined within a housing of either the first turbine orthe second turbine, the barrel valve body being rotatable about a valveaxis to selectively permit or block flow to the second turbine inletfrom the first exhaust gas passage and/or bypass gas passage. The barrelvalve body is a form of valve rotor of a rotary valve. In otherembodiments of the invention the exhaust flow control valve may comprisea rotary valve of a type other than a valve comprising a barrel valvebody rotatably housed within a valve chamber defined within a housing ofeither the first turbine or the second turbine.

Embodiments of the invention may comprise a pressure release valve forventing gas from either the exhaust bypass gas passage or the valvechamber upstream of the second turbine inlet. The pressure release valvemay be operable to maintain pressure within the exhaust gas bypasspassage and/or valve chamber within a desired range and/or below adesired maximum and/or at a desired value. The pressure release valvemay be biased into a closed position and operates to open when pressurewithin the bypass gas flow passage or valve chamber increases above athreshold sufficient to move the valve into an open position against thebiasing force.

The rotary valve may be housed within a housing which is separate to orintegral with either one of the first and second turbine housings.

The first and second turbines may have a common turbine housing, therotary valve being housed within said common turbine housing eitherupstream or downstream of the first turbine inlet.

The valve rotor may comprise an arcuate surface which at least partiallycovers the first and/or second, and/or third port when the valve rotoris in a position in which the respective port is at least partiallyobstructed. The arcuate surface may for instance define a sector of acylinder.

The valve rotor may be rotatably supported at first and second endsthereof for rotation about said axis. An electric, pneumatic, hydraulicor other form of actuator may be provided for effecting rotation of thevalve rotor. For instance the rotary valve or actuator may be adapted toreceive control signals from a controller such as an engine control unit(ECU).

The turbocharger system may include at least one sensor from thefollowing group, a sensor for detecting or determining the speed ofrotation of the first turbine and/or the second turbine, a sensor fordetecting or determining the boost pressure generated by the firstand/or second turbocharger; a sensor for detecting or determining theback pressure in the exhaust gas bypass path; a sensor for detecting ordetermining pressure within the rotary valve chamber; a sensor fordetecting or determining the rotational speed of an engine to which theturbocharger system is operably connected; and a sensor for detecting ordetermining load on an engine to which the turbocharger system isoperably connected.

A second turbocharger exhaust bypass flow passage may be provided,communicating between the outlet of the second turbocharger and theinlet of the second turbocharger downstream of said junction, and abypass flow control valve provided for selectively permitting exhaustgas flow to bypass said second turbine.

A turbocharged internal combustion engine including a turbochargersystem according to the present invention may include an exhaust gasre-circulation system comprising a re-circulation gas flow path from anexhaust side to an air intake side of the engine, and an exhaust gasre-circulation valve provided in said path for controllingre-circulation of exhaust gas to the air intake side of the engine,wherein the rotary valve is operable to modulate back pressure in theexhaust gas flow to assist in the exhaust re-circulation.

The present invention also provides a turbocharger, comprising a turbinewheel mounted for rotation within a turbine chamber defined by a turbinehousing, the housing defining a first inlet passage, a second inletpassage, and a third passage communicating with said turbine chamber,the first, second and third passages meeting at a junction, wherein arotary valve is located at said junction, the rotary valve comprising avalve rotor which is rotatable about a valve axis to selectively permitor block exhaust gas flow to the third inlet passage from either thefirst inlet passage and/or second inlet passage.

The valve rotor may rotate within a valve chamber defined by thehousing, the valve chamber having a first inlet port communicating withthe first inlet passage, a second inlet port communicating with thesecond inlet passage, and an outlet port communicating with the thirdinlet passage.

The valve rotor may be rotatable about said axis into a position inwhich each of the three ports is at least substantially unobstructed.The valve rotor may be rotatable about said axis into at least oneposition in which the first inlet port and the outlet port areunobstructed, and the second inlet port is at either partially or fullyobstructed. The valve rotor may rotatable about said axis into at leastone position in which the first inlet port is unobstructed, the secondinlet port is fully obstructed and the outlet port is either partiallyor fully obstructed. The valve rotor may rotatable into at least oneposition in which the second inlet port is unobstructed, the first inletport is fully obstructed, and the outlet port is partially or fullyobstructed. The valve rotor may be rotatable into a position in whichthe outlet port is fully obstructed.

A pressure release valve for controlling or limiting pressure within thesecond inlet passage and/or the valve chamber.

The first turbocharger may include a first air-compressor situated in afirst air passage, and the second turbocharger may include a secondair-compressor situated in said first air flow passage upstream of saidfirst compressor. A bypass air flow passage may communicate with thefirst air flow passage upstream and downstream of the firstair-compressor, and an air flow bypass valve may be operated to controlthe air flow through the first compressor and the bypass air flowpassage.

The present invention also provides methods of operating theturbocharger system according to the present invention. Operating modesmay include a normal fired mode for controlling the exhaust gas flow tothe first and second turbines, a braking mode (non-fired mode) in whichthe rotary valve is operated to provide a braking back pressure in theexhaust system, and an exhaust gas heating mode (fired operating mode)in which the valve is operated to restrict exhaust gas flow to therebyheat the exhaust gas flow. In the exhaust gas heating mode, theturbocharger system may be part of a system comprising an exhaustafter-treatment system.

Specific embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic illustration of a two-stage turbocharging systemaccording to a first embodiment of the present invention;

FIG. 2 is a schematic cross-section through an exhaust gas control valveof the two-stage turbocharging system of FIG. 1;

FIG. 3 is a perspective illustration of a valve rotor according to anembodiment of the present invention;

FIGS. 4 a-4 c schematically illustrate a fired mode of operation of thepresent invention;

FIG. 5 schematically illustrates a braking mode of operation of thepresent invention;

FIG. 6 schematically illustrates an exhaust heating mode of operation ofthe present invention;

FIG. 7 is an end view of a turbocharger of a turbocharging system inaccordance with the present invention;

FIGS. 8 a to 8 e are cross-sections of the turbocharger of FIG. 7illustrating a valve rotor position in various operating modes;

FIG. 9 schematically illustrates an alternative exhaust control valve inaccordance with an embodiment of the present invention;

FIG. 10 schematically illustrates application of the turbochargingsystem of FIG. 1 to an engine with an EGR system;

FIG. 11 illustrates a modification of the turbocharger system of FIG. 1in accordance with the present invention;

FIG. 12 is a schematic illustration of a two-stage turbocharging systemaccording to a further embodiment of the present invention;

FIGS. 13 a to 13 c schematically illustrate a fired mode of operation ofthe turbocharging system of FIG. 12;

FIG. 14 schematically illustrates a braking mode of operation of theturbocharging system of FIG. 12; and

FIG. 15 schematically illustrates an exhaust heating mode of operationof the turbocharging system of FIG. 12.

Referring first to FIG. 1, the schematically illustrated sequential twostage turbocharging system comprises a relatively small high pressure(HP) turbocharger 1 and a relatively large low pressure (LP)turbocharger 2 connected in series to the exhaust manifold 3 of aninternal combustion engine (not shown) such as a diesel engine. The HPturbocharger 1 comprises a relatively small exhaust turbine 4 and arelatively small compressor 5. The LP turbocharger 2 comprises arelatively large exhaust turbine 6 and a relatively large compressor 7.

An exhaust gas flow control valve 8 is located within the turbinehousing 9 of the low pressure turbocharger 2 to control exhaust gas flowto the LP turbine exhaust gas inlet path 10. The exhaust flow controlvalve 8 is operable to control flow to the LP turbine inlet path 10 viatwo possible exhaust gas flow paths. A first exhaust gas flow path 11delivers exhaust gas from the exhaust manifold 3 to the LP turbine inletpath 10 via the HP turbine 4 and control valve 8. A second, bypass, flowpath 12 delivers exhaust gas from the exhaust manifold 3 to the LPturbine inlet path 10 via valve 8 only, bypassing the HP turbine 4.Exhaust gas leaves the LP turbine 6 via exhaust outlet path 13 fromwhere it may be fed to a conventional exhaust system which may include aconventional exhaust after-treatment system 13 a. The after-treatmentsystem 13 a may be one of a variety of types of after-treatment system,including conventional systems generally known to one of ordinary skillin the art. Types of after-treatment systems contemplated include thosedesigned to remove particulates, nitrogen-oxide compounds, and otherregulated emissions.

As will be described in more detail below, the control valve 8 accordingto the present invention is operable to permit/block (and to modulate)exhaust gas flow to the LP inlet path 10 from either one or both of thetwo flow paths 11 and 12.

The turbocharging system delivers compressed air to the engine(including any after cooler as appropriate) via an air inlet 14 to theLP compressor 7. An air flow control valve 15 is located within thecompressor housing 16 of the LP turbocharger 2 to control the flow fromthe LP turbine outlet path 17 to the engine (after-cooler etc). The airflow control valve 15, which may for instance be a conventionalbutterfly valve (or other valve type such as a rotary valve, gate valve,flap valve, poppet etc), is operable to control air flow along twopossible flow paths, a first flow path 18 via the HP compressor 5, and asecond, bypass, flow path 19 which allows the air flow to bypass the HPcompressor 5. The air flow control valve 15 can thus be controlled (forinstance by the engine management system electronic control unit-ECU) toallow air flow to bypass the HP turbocharger 1 at the same time as theexhaust gas control valve 8 is operated to allow exhaust gas flow to theLP turbocharger 2 to bypass the HP turbocharger 1. This is described inmore detail below.

FIG. 2 is a schematic cross-section through the exhaust control valve 8which is a rotary valve comprising a valve rotor 19, having an axis ofrotation X (extending into the paper with respect to FIG. 2) within asubstantially cylindrical valve chamber 20 defined within the LP turbinehousing 9 at the junction of the LP turbine inlet passage 10, theexhaust gas flow path 11 from the HP turbine 4, and the bypass flow path12 from the exhaust manifold 3. The axis extends generally transverse tothe flow paths 10, 11 and 12. The valve rotor 19 is formed as a sectorof a cylinder of substantially constant cross-section along its lengthdefining a valve passage 21 through the valve chamber 20. The radiallyouter surface 19 a of the rotor forms an arc of a cylinder so as torotate freely within the cylindrical valve chamber 20. This general formof valve is sometimes referred to as a rotary plug valve.

Rotation of the valve rotor 19 about the axis X rotates the valvepassage 21 and brings the rotor surface 19 a into alignment with valveports 10 a, 11 a and 12 a of the paths 10, 11 and 12 respectively topartially or completely block the flow through the respective port/path.In other words, rotation of the valve rotor 19 effectively rotates thevalve passage 21 to permit exhaust gas flow to the LP turbine inlet path10 through the valve chamber 20 from either one, or both, of the exhaustflow paths 11 and 12, or to completely block the flow through the valvechamber 20.

A perspective view of a one embodiment of a valve rotor 19 is shown inFIG. 3. At either axial end of the rotor 19 is a stub shaft 19 bdefining the axis X and permitting the valve rotor to be rotatablymounted within a valve body defining the valve chamber 20 (the valvebody is not shown in FIG. 3). The particular rotor illustrated has aninternal bore 19 b to reduce the weight of the rotor. Appropriatebearing arrangements (not shown) for mounting of the rotor within thevalve body, and appropriate valve actuating mechanisms (not shown), willbe known to the skilled person. For instance the actuator may be anelectric actuator, such as for example a stepper motor or other rotaryelectric actuator, or may comprise a pneumatic or hydraulic actuator orany other form of actuator. An actuator may be directly connected to oneof the valve rotor shafts 19 b, or to a valve rotor spindle (not shown)which may extend from one of the axels. The actuator may be directlyconnected to the rotor or connected to the rotor via a gear box or thelike. Various possible coupling arrangements will be apparent to theappropriately skilled person.

The movement and positioning of the valve rotor will typically becontrolled by the ECU, according to one or more control regimes. Forinstance, the position of the valve may be controlled in response toengine speed and/or load, the speed of the HP and/or LP turbines, or theboost pressure produced at the engine inlet manifold by the turbochargersystem.

Exemplary modes of operation of the turbocharging system including theexhaust gas control valve 8 are described below with reference to FIGS.4 a to 4 c, 5 and 6. In these figures valve rotor 19 is shownschematically with a slightly different cross-sectional profile, whichdefines a different valve passage configuration, to that illustrated ineither FIG. 2 or FIG. 3, but the principal of construction and operationis the same.

Referring first to FIG. 4 a this illustrates a position of the valverotor 19 and valve passage 21 appropriate for exhaust flow control atlow engine speeds and/or loads when there is low exhaust mass flow. Thebypass exhaust gas flow path port 12 a is closed by the surface 19 a ofthe valve rotor 19 so that all of the exhaust gas flow from the enginemanifold 3 to the LP turbine inlet path 10 flows along the flow path 11through the HP turbine 4. The air flow bypass valve 15 will also beclosed, or substantially closed, to force air flow through the HPcompressor 5 (in practice it is beneficial to close the compressorbypass valve 15 before the turbine bypass is closed to provide load onthe HP compressor which will prevent HP turbocharger overspeed as theturbine bypass is closed). Due to the relatively small size of the HPturbine 4 the gas flowing through it reaches a relatively high speed andthus rotates the turbine 4 (and consequentially HP compressor 5) at arelatively high speed, thereby producing substantial boost pressuredespite a relatively low exhaust mass flow rate. Because of itsrelatively large size the LP turbine 6 rotates very little so that theLP compressor 7 produces only marginal boost.

With the valve rotor 19 in the position shown in FIG. 4 a, the divisionof work between the HP and the LP turbines is a function of the relativeflow areas of each turbine. The HP turbine is providing the majority ofthe work and operating at a much higher expansion ratio than the largerLP turbine. If engine speed and/or load increases with the valve rotorin the portion shown in FIG. 3 a, the expansion ratio of both turbineswill increase, but the HP turbocharger will continue to provide most ofthe boost pressure (provided its effective expansion ratio limit is notexceeded).

As the engine speed and/or load increases, the valve rotor 19 may berotated to uncover part, or all, of the port 12 a of the bypass exhaustgas path 12 to permit at least a portion of the exhaust gas flow tobypass the HP turbine. FIG. 4 b illustrates the valve rotor rotated to aposition in which the port 12 a of the exhaust bypass gas path 12 ispartially uncovered, and FIG. 4 c shows the valve rotor 19 rotated to aposition in which the port 12 a of the bypass exhaust gas path 12 iscompletely uncovered. By controlling the position of the valve rotor 19between the two extremes shown in FIGS. 4 a and 4 c, it is possible inaccordance with the present invention to modulate the bypass gasflowthrough the bypass gas path 12. For instance, as the engine speed beginsto rise from a low speed and/or load condition, the valve rotor 19 maybe rotated to begin to open the bypass exhaust gas port 12 a to permitsome exhaust gas flow to bypass the HP turbine so that an increasingamount of work is done by the LP turbocharger as the engine speed and/orload rises.

The precise position of the valve rotor 19 may be controlled inaccordance with a variety of different operating control strategies. Forexample, the valve 8 may be operated to maintain a particular expansionratio across the HP turbine, either to maintain the HP turbine at aconstant expansion ratio or at an expansion ratio within an acceptablerange for particular operating conditions of the engine. The valve 8could alternatively or additionally be operated in order to maintain theHP turbine speed within a certain range, or below a certain maximum toprevent over-speed. According to another possible control strategy, thevalve 8 could be operated to generate a desired boost pressure at theengine intake manifold or to maintain the boost pressure within adesired range (e.g. above a minimum and/or below a maximum). Theprovision of appropriate sensors, such as turbocharger speed or boostpressure sensors, to provide appropriate control signals to the ECU willbe straightforward as will be appreciated by the appropriately skilledperson. The sensors might typically include sensors for monitoringengine speed and/or load, turbocharger speed, boost pressure produced byeach turbocharger, boost pressure generated at the engine intake andback pressure generated within the exhaust flow path upstream of thecontrol valve 8.

As the engine speed and/or load rises and the valve rotor 19 is rotatedfurther towards the position shown in FIG. 4 c in which the bypass gaspassage is fully open; work done by the LP turbocharger relative to theHP turbocharger increases. Depending upon the particular controlstrategy for the bypass exhaust gas flow modulation, the overallpressure ratio of the turbocharger system may for instance rise orremain constant as the expansion ratio across the larger LP turbineincreases.

It will be appreciated that as the bypass exhaust gas path port 12 a isopened by rotation of the valve rotor 19, the HP compressor bypass valve15 may also be opened as an increasing amount of boost is provided bythe LP compressor. The overall boost pressure produced by theturbocharging system may rise, or may remain constant, as the bypassexhaust gas path 12 is opened depending upon the particular controlregime for the control valve 8 and bypass valve 15.

At high engine load and/or speed, the valve rotor 19 is moved to theposition shown in FIG. 4 c in which the bypass exhaust gas path 12 isfully open, the turbocharging system again functions effectively as asingle turbocharger system, with virtually all of the work now beingdone by the larger LP turbocharger. At this point the HP compressorbypass valve 15 will typically be fully open to bypass the HP compressor5. There will, however, still be some exhaust gas flow through the HPturbine as there will be a pressure difference across it determined bythe relative sizes of the flow passages 11 and 12. Although this wouldproduce negligible work, it will nevertheless ensure that the HP turbinecontinues to rotate to help provide a smooth transfer of work withlittle turbo-lag in the HP turbocharger as engine conditions change andthe valve 8 is operated to reduce the bypass flow, transferringcompression work to the HP turbocharger.

The present invention thus provides a turbocharger system including anexhaust gas flow control valve which can be operated precisely tomodulate the exhaust gas flow to the HP and LP turbochargers in variedoperating conditions, and in accordance with various possible controlregimes. This may be regarded as a normal fired mode operation of theturbocharging system according to the present invention. In accordancewith the present invention the turbocharging system may in someembodiments be operated in one or both of two further modes, namely anengine braking mode and an exhaust gas heating mode.

Engine brake systems of various forms are widely fitted to vehicleengine systems, in particular to compression ignition engines (dieselengines) used to power large vehicles such as trucks. The engine brakesystems may be employed to enhance the effect of the conventionalfriction brakes acting on the vehicle wheels or, in some circumstances,may be used independently of the normal wheel braking system, forinstance to control down hill speed of a vehicle. With some engine brakesystems, the brake is set to activate automatically when the enginethrottle is closed (i.e. when the driver lifts his foot from thethrottle pedal), and in others the engine brake may require manualactivation by the driver, such as depression of a separate brake pedal.

In one form of conventional engine brake system an exhaust valve in theexhaust line is controlled to substantially block the engine exhaustflow when braking is required. This produces an engine braking torque bygenerating a high backpressure that increases the work done on theengine piston during the exhaust stroke.

In accordance with an embodiment of the present invention, the exhaustflow control valve 8 can be operated to provide exhaust braking,obviating the need to provide a dedicated exhaust brake valve. Whenoperating the exhaust gas control valve 8 in an engine braking mode inaccordance with the present invention, the valve rotor 19 is rotatedclockwise to completely block the port 11 a of the gas flow path 11 fromthe HP turbine 5, and to at least partially block the LP turbine inletport 10 a as shown in FIG. 5. Blocking the inlet 10 to the LP turbineobstructs exhaust gas flow through the turbocharger system therebygenerating back pressure for engine braking.

The amount of back pressure generated can be modulated by appropriatecontrol of the position of the valve rotor 19 to vary the degree towhich the LP turbine inlet valve port 10 a is obstructed. For instance,the dotted lines in FIG. 5 show the rotor position 19 to completelyclose the LP turbine inlet port 10 a. At least some leakage flow throughthe turbocharger system is however desirable even at maximum braking toprevent excessive back pressure in the engine. This could be provided byprovision of an alternative leakage flow path (such as a blow-off valvein the gas flow path 12 upstream of the exhaust control valve 8 orcombined with the valve 8 itself. Alternatively a maximum gas flow maybe provided by allowing at least some gas flow through the valve chamber20 to the LP turbine inlet 10. It may be beneficial to provide formodulation of the leakage flow (for instance by varying the area of theleakage flow path through the valve chamber 20, or the leakage flowthrough a pressure relief valve) to for instance maintain a constant orother desired pressure characteristic over the engine speed range underbraking. This may for instance be controlled by the ECU. Othercharacteristics such as turbocharger speed of either the HP or the LPturbocharger could be maintained at a safe level by such modulation. Forinstance, if the leakage flow path is a path past the valve rotor, themodulation may be achieved by adjusting the position of the valve rotor.In other embodiments, leakage flow may be provide by providing a “hardstop” preventing the valve rotor 19 from completely closing the LPturbine inlet port 10 a.

Since in braking mode as illustrated in FIG. 5 the valve rotor 19completely blocks the outlet from the HP turbine, any gas flow throughthe turbocharger system in the engine braking mode will flow onlythrough the LP turbine. This ensures that there will be reduced boostpressure generated during exhaust braking, and will also ensure that theHP turbocharger cannot over-speed, bearing in mind that an exhaust brakeis often applied when a vehicle is travelling long distances down hillduring which there is only light load on the engine but very high enginespeeds can be reached. It would also be possible to operate the controlvalve 8 in an engine braking mode by rotating the rotor 19 in theopposite direction so as to completely block the bypass gas flow port 12a and at least partially block the LP inlet port 10 a (a position asshown for instance in FIG. 6). However, this is less desirable forbraking because of unwanted boost pressure that might be generated bythe HP turbine and the potential risk of the HP turbine over speeding.

Whereas the exhaust braking mode is typically a non-fired operating mode(no fuel is supplied to the engine during exhaust braking), the controlvalve 8 may also be operated to restrict exhaust gas flow in a firedmode (in which fuel is supplied to the engine for combination) to raiseexhaust gas temperature in order to regenerate a catalytic exhaustafter-treatment.

Catalytic exhaust after-treatment system performance is directly relatedto the temperature of the exhaust gas that passes through it. Fordesired performance the exhaust gas temperature must be above athreshold temperature (typically lying in a range of about 250° C. to370° C.) under all engine operating conditions and ambient conditions.Operation of the after-treatment system below the threshold temperaturerange will cause the after-treatment system to build up undesirableaccumulations which must be burnt off in a regeneration cycle to allowthe after-treatment system to return to designed performance levels. Inaddition, prolonged operation of the after-treatment system below thethreshold temperature without regeneration will disable theafter-treatment system and cause the engine to become non-compliant withgovernment exhaust emission regulations.

For the majority of the operation range of a diesel engine for instance,the exhaust gas temperature will generally be above the requiredthreshold temperature. However, in some conditions, such as light loadconditions and/or cold ambient temperature conditions, the exhaust gastemperature can often fall below the threshold temperature.

In engine operating conditions, such as light load conditions, in whichexhaust temperature might otherwise drop below the required thresholdtemperature the exhaust control valve can be operated in an exhaust gasheating mode to restrict exhaust gas flow thereby reducing the airflowcooling effect and increasing exhaust gas temperature.

A position of the valve rotor 19 and valve passage 21 appropriate to anexhaust gas heating mode is illustrated in FIG. 6. The valve rotor 19blocks flow through the bypass exhaust gas port 12 a but leaves the gasflow path through the HP turbine unobstructed. However, the LP turbineinlet gas port 10 a is partially obstructed to restrict flow to the LPturbine 6 (at the same time the compressor bypass valve 15 may be closedso that the incoming air flows through the HP compressor 5).

The effect of operating the valve in this way is to reduce the gas flowthrough the engine for any given fuel supply level (whilst maintainingsufficient air flow for combustion) in order to increase the exhaust gastemperature for instance to a level required for efficient operation andregeneration of a catalytic exhaust after-treatment system. As mentionedabove, this may for example be appropriate in such conditions as lightload conditions and/or cold ambient temperature conditions. The heatingeffect can be modulated by precise control of the valve body 19 to varythe degree of obstruction of the LP inlet path port 10 a.

As with other modes of operation of the exhaust control valve 8, theexhaust gas heating mode of operation of the valve 8 will typically becontrolled by the ECU. For instance, the after-treatment system mayinclude a temperature detector for determining the temperature withinthe system. The temperature detector may directly determine thetemperature through a sensor, or may determine the temperature throughcalculations and/or iterations in an algorithm or software routine. Thetemperature detector may determine the temperature within the system andprovide a signal to the ECU to facilitate control of the exhaust gascontrol valve 8 to effect a change in the exhaust gas temperature asneeded. The temperature determination could be made within theafter-treatment system itself, or at other locations such as the outletof the LP turbine.

The temperature determinations may be made at regular time intervals,for example a plurality of closely timed intervals, or could forinstance be effectively continuous. The ECU may be programmed to operatethe exhaust control valve 8 in an exhaust gas heating mode whenever thetemperature in the exhaust system is determined to have fallen below athreshold temperature.

FIG. 7 and FIGS. 8 a to 8 e illustrate an LP turbocharger of aturbocharging system according to the present invention in which theexhaust control valve 8 is housed within a suitably adapted LP turbinehousing 30. The inlet to the turbine housing is modified to define acontrol valve housing 31. A valve rotor spindle 32 extends from thehousing for connection to an appropriate valve actuator (not shown).Also visible in FIG. 7 is an inlet manifold 33 for connection to theoutlet of an HP turbine, and the LP turbine outlet 34.

FIGS. 8 a to 8 e are cross-sections of the LP turbine housing of FIG. 7taken on the line A-A of FIG. 7. These figures reveal a part of theturbine inlet path 35 as well as the HP turbine inlet path 36, exhaustbypass gas path 37 and respective inlet flanges 33 and 38. It will beappreciated that the gas flow paths 35, 36 and 37 correspond to the gasflow paths 10, 11 and 12 described above respectively. The valve rotor39 is shown in cross-section and has a profile generally conforming tothe profile of the valve rotors described above. In FIGS. 8 a and 8 bthe valve rotor 39 is shown in a position corresponding generally to theposition of the valve rotor shown in FIGS. 4 a and 4 c appropriate tonormal fired mode operation. FIG. 8 c shows the valve rotor in aposition corresponding to the position shown in FIG. 6 appropriate foroperation in an exhaust gas heating mode. FIGS. 8 d and 8 e show thevalve rotor in positions corresponding generally to positionsillustrated in FIG. 5 appropriate for an engine braking operating mode.

A modification of the exhaust flow control valve described above andwhich incorporates a pressure release valve is schematically illustratedin FIG. 9. The rotary valve is substantially the same as the rotaryvalves above comprising a valve rotor 19 rotatably mounted within avalve chamber 20 to selectively cover/uncover gas flow ports 10, 11 aand 12 a. The modification comprises the inclusion of a pressure reliefvalve 50 which comprises a spring biased valve member 51 which seatswithin a valve passage 52. As pressure within the rotary valve chamber20 rises above a threshold pressure, the pressure relief valve member 51is forced off its valve seat to allow exhaust gas to vent into theexhaust gas flow downstream of the LP turbine via passages 52 and 53. Inan engine braking mode, as illustrated, the rotor 19 may be moved tocompletely block flow to the LP turbine via LP turbine inlet port 10 a,the pressure relief valve 50 ensuring that the back pressure does notrise above a predetermined maximum. In a more sophisticated modificationof the control valve (not illustrated) the pressure relief valve 50could be controlled via the ECU to modulate the back pressure within theexhaust system to modulate the braking effect. This could beadvantageous if it is found difficult in practice to accurately controlthe position of the rotary valve rotor 19 to the degree required toprovide precise modulation of the braking effect. It will be appreciatedthat the exact form of the pressure relief valve may vary from thatillustrated, and could for instance be a butterfly valve, flap valve,poppet valve or any other of a variety of suitable valves which will bereadily apparent to the appropriately skilled person.

The turbocharger system of the present invention may also be operated toassist exhaust gas recirculation (EGR). In an EGR system a portion ofthe exhaust gas taken from the exhaust manifold is reintroduced into theinlet manifold of the engine for further combustion with a view toreducing engine emissions. Incorporation of the turbocharging system ofFIG. 1 in an engine including an EGR system is schematically illustratedin FIG. 10. The illustrated EGR system includes an EGR recirculationpath 40 flowing a portion of the exhaust gas to the intake manifold 41of the engine via an EGR cooler 42. Flow through the exhaustrecirculation path 40 is controlled by an EGR control valve 43. The EGRcontrol valve 43 may be any one of a variety of conventional typescommonly used in such an application, including butterfly valves, flapvalves, rotary valves etc.

With modem highly efficient turbocharger systems, the boost pressure ofthe inlet manifold can often exceed the exhaust gas pressure at theexhaust manifold making the reintroduction of the recirculated exhaustgas to the inlet manifold problematical, for instance requiringdedicated EGR pumps etc. With the present invention, the exhaust gascontrol valve can be operated in such a way as to effectively reduceturbocharging efficiency below the maximum that could be achieved forany given engine operating condition in order to maintain back pressureat a level necessary to facilitate exhaust gas recirculation. In otherwords, the exhaust control valve 8 may be operated in such a way as tooptimise engine intake and exhaust manifold conditions for exhaust gasrecirculation in order to reduce emissions whilst at the same timeminimising the air-fuel ratio for better fuel consumption.

A modification of the turbocharging system of FIG. 1 in which the LPturbine is provided with a wastegate valve 60 is shown in FIG. 11. Thewastegate valve 60 may have any conventional form (such as for instancea poppet valve) as is well known in the art of turbocharging. Thewastegate valve 60 may similarly be operated in a conventional way, forinstance by a pneumatic actuator or electric actuator (not shown) toallow some of the exhaust gas flow to bypass the LP turbine as boostpressure in the LP compressor (or at the engine intake manifold) reachesa threshold value. Alternatively, or additionally, the wastegate valve40 could for instance be operated under the control of the ECU inresponse to a control regime programmed into the ECU, for example toallow some exhaust gas flow to bypass the LP turbine in order to controlthe speed of the LP turbocharger or boost pressure produced by the LPturbine or the turbocharging system as a whole. Various methods andmodes of operation of the wastegate valve will be apparent to theskilled person.

In the schematic illustrations of FIGS. 1 and 4 the exhaust controlvalve is shown downstream of the HP turbine 5. It will be appreciatedhowever that in other embodiments of the invention the control valvecould be positioned upstream of the HP turbine 4 as schematicallyillustrated in FIG. 12. FIG. 12 is based on FIG. 1 and where aappropriate the same reference numerals will be used. In accordance withthis embodiment of the invention, a modified exhaust flow control valve80 is positioned upstream of the HP turbine 4 and likewise upstream of aHP bypass flow path 81. A first exhaust gas flow path 82 deliversexhaust gas from the exhaust manifold 3 to the LP turbine inlet path 83.The first exhaust gas path 82 has a first portion 82 a communicatingbetween the exhaust manifold 3 and the control valve 80, a secondportion 82 b communicating between the control valve 80 and the inlet ofthe HP turbine 4, and a third portion 82 c communicating between theoutlet of the HP turbine 4 and the LP turbine inlet path 83.

The control valve 80 may have substantially the same construction ascontrol valve 8 described above, but in this case the three valve threevalve ports comprise a single inlet port and two outlet ports. The portsare illustrated in FIGS. 13 a to 13 c, 14 and 15 which illustrate modesof operation of the turbocharging system of FIG. 12 which correspond tothe modes of operation of the turbocharging system of FIG. 1 asillustrated in FIGS. 4 a to 4 c, 5 and 6 respectively.

Referring first to FIGS. 14 a to 14 c, the control valve 80 comprises avalve rotor 19 rotating within a valve chamber 20 to selectively openand close three valve ports, namely a valve inlet port 84 whichcommunicates with the gas flow path 82 a, a first outlet port 85 whichcommunicates with gas flow path 82 b, and a second outlet port 86 thatcommunicates with bypass gas flow path 81. The position of the valverotor 19 illustrated in FIG. 13 a is appropriate for exhaust flowcontrol at low engine speeds and/or loads where there is a low exhaustmass flow. The bypass exhaust gas flow path port 86 is closed by therotor 19 so that all of the exhaust gas flow from the engine manifold 3to the LP turbine 6 must flow along the first flow path 82 comprisingportions 82 a and 82 b, i.e. flowing through the HP turbine 4.

As the engine speed and/or load increases, the valve rotor 19 may berotated to uncover part, or all, of the valve port 86 to permit at leasta portion of the exhaust gas flow to flow along the bypass gas path 81thereby bypassing the HP turbine 4. FIG. 13 b illustrates the valverotor 19 in a position in which the port 86 is partially uncovered, andFIG. 13 c shows the valve rotor 19 in a position in which the port 86 iscompletely uncovered. By controlling the position of the valve rotor 19between the two extremes shown in FIGS. 13 a and 13 c it is possible inaccordance with the present invention to modulate the bypass gas flowthrough the bypass gas flow path 81.

FIGS. 14 and 15 illustrate operation of the control valve 80 in abraking mode and exhaust gas heating mode respectively. In the brakingmode, the rotor 19 is rotated to block flow to the HP turbine 4 viavalve port 85, and to at least partially block flow to the LP turbine 6via the bypass gas flow path 86 by at least partially covering the valveinlet port 84. The amount of braking back pressure generated can bemodulated by controlling the precise position of the valve rotor 19. Forinstance, the dotted lines in FIG. 15 show the rotor 19 in position tocompletely close the valve port 85. In this case, it may be preferableto provide a path for a leakage flow to prevent back pressure exceedinga desired limit (which could be in the same way as discussed above inrelation to FIG. 5). FIG. 15 shows position of the rotor appropriate toexhaust gas heating, which is a fired operating mode of the engine. Herethe rotor 19 is positioned to block exhaust gas flow to the LP turbine 6via the valve port 86 so that all exhaust gas flows to the LP turbine 6via the HP turbine (through port 85). However, once again the rotor 19at least partially covers the valve inlet port 84 to restrict exhaustgas flow from the exhaust manifold 3 to thereby heat the exhaust gas inthe same way as described above in relation to FIG. 6.

It will be appreciated that features and modifications that can be madeto the turbocharging system of FIG. 1 may also be made to theturbocharging system of FIG. 13, such as for instance the operation ofthe LP and HP compressors, the inclusion of a wastegate around the LPturbine, and application of the system to an engine including theinclusion of an exhaust gas re-circulation system.

In the illustrated embodiments of the invention (in which the controlvalve 8 is downstream of the HP turbine) the valve is convenientlylocated in the housing of the LP turbine. It will be appreciated that inalternative embodiments of the invention (in which the valve is eitherupstream or downstream of the HP turbine) the exhaust flow control valvemay be housed in a separate valve housing which is not part of either ofthe two turbochargers. Such an embodiment would for instance allowretrofitting of the control valve 8 to a two-stage turbocharging system.In other embodiments of the invention, particularly in embodiments inwhich the control valve is located upstream of the HP turbine inlet, theexhaust flow control valve could for instance be housed within the HPturbine housing. In yet other embodiments, the HP turbine and LPturbines may be combined in a common housing, the exhaust control valvebeing located within that common turbine housing.

Turbines may be of a fixed or variable geometry type. Variable geometryturbines differ from fixed geometry turbines in that the size of theinlet passageway can be varied to optimise gas flow velocities over arange of mass flow rates so that the power output of the turbine can bevaried to suite varying engine demands. For instance, when the volume ofexhaust gas being delivered to the turbine is relatively low, thevelocity of the gas reaching the turbine wheel is maintained at a levelwhich ensures efficient turbine operation by reducing the size of theannular inlet passageway. Turbochargers provided with a variablegeometry turbine are referred to as variable geometry turbochargers.

In one known type of variable geometry turbine, an axially moveable wallmember, generally referred to as a “nozzle ring”, defines one wall ofthe inlet passageway. The position of the nozzle ring relative to afacing wall of the inlet passageway is adjustable to control the axialwidth of the inlet passageway. Thus, for example, as gas flow throughthe turbine decreases, the inlet passageway width may be decreased tomaintain gas velocity and optimise turbine output.

Another known type of variable geometry turbine is the “swing vane”type. This comprises a variable guide vane array with adjustable guidevanes located in the turbine inlet passageway. Each vane is pivotableabout a respective pivot axis extending across the inlet parallel to theturbine axis. A vane actuating mechanism is provided which is linked toeach of the vanes and is displaceable in a manner which causes each ofthe vanes to move in unison, such a movement enabling thecross-sectional area of the inlet, and also the angle of approach of thegas turbine wheel, to be controlled.

Although two stage turbocharging systems comprising fixed geometryturbines may in some respects provide an alternative to the use ofrelatively complex and expensive variable geometry turbochargers, one(or even both) of the turbochargers of a two stage turbocharging systemaccording to the present invention could be a variable geometryturbocharger (of any type). This may be desirable for instance tofurther improve control over the turbocharging system and the ability tooptimise turbocharging performance across a wide range of engineconditions.

In the above described embodiments of the invention there is a single HPturbine. However, it will be appreciated that a turbocharging systemaccording to the present invention could for instance include twoparallel HP turbines. For example, each of two HP turbines could receivean exhaust gas flow from a respective bank of cylinders from amulti-cylinder engine (for instance each receiving exhaust gas from onebank of a “V” configured engine). In such embodiments, with a singleexhaust flow control valve 8 located downstream of the HP turbines, theoutlet from each HP turbine can be combined upstream of the exhaust flowcontrol valve. There may be a single LP turbine located downstream ofthe valve, or the flow may be split between two (or more) LP turbinesdownstream of the valve.

In embodiments comprising more than one HP turbine, HP turbines can belinked to a common HP compressor or to separate respective HPcompressors.

Alternatively, rather than providing two separate HP turbines to receiveexhaust gas flow from two separate banks of engine cylinders, a singletwin entry HP turbine could be included in a turbocharger systemaccording to the present invention. Moreover, in a turbocharger systemaccording to the present invention comprising one or more HP turbines,each of the HP turbines could be configured as a twin-entry turbine.

Similarly, it will be appreciated that a turbocharging system inaccordance with the present invention could have more than one set ofsequentially connected turbochargers operating in parallel. Forinstance, a first turbocharging system generally as described abovecould receive an exhaust gas flow from a first set of cylinders of amulti-cylinder combustion engine, and a second sequential turbochargingarrangement as described above could receive exhaust gas flow from asecond set of cylinders of the engine (each “set” could comprise asingle cylinder).

It will further be appreciated that the present invention is not limitedto a two stage sequential turbocharging system, but could be embodied ina turbocharging system comprising more than two turbine stages connectedin series.

Other applications and modifications of the invention as described abovewill be apparent to the appropriately skilled person.

1. A turbocharger system comprising: a first relatively small turbocharger; a second relatively large turbocharger; the first turbocharger comprising a first exhaust turbine situated in a first exhaust passage; the second turbocharger including a second exhaust turbine situated in said first exhaust passage downstream of said first turbine; an exhaust bypass flow passage communicating with the first exhaust flow passage upstream and downstream of the first turbine; and a rotary valve located at a junction of the bypass flow passage and the first exhaust gas flow passage, the rotary valve comprising a valve rotor which is rotatable about a valve axis to selectively permit or block flow to the second turbine inlet from the first exhaust gas passage and/or bypass gas passage.
 2. A turbocharger system according to claim 1, wherein the valve rotor rotates within a valve chamber which has a first port communicating with the first exhaust gas passage, a second port communicating with the bypass exhaust gas passage, and a third port communicating with the second turbine.
 3. The turbocharger system according to claim 2, wherein the valve rotor is rotatable about said axis into a position in which each of the three ports is at least substantially unobstructed.
 4. A turbocharger system according to claim 2, wherein the valve rotor is rotatable about said axis into one of a plurality of positions in which the first port and third port are unobstructed and the second port is at least a partially obstructed.
 5. A turbocharger system according to claim 2, wherein the valve rotor is rotatable into a position in which the second port is fully obstructed.
 6. A turbocharger system according to claim 2, wherein the valve rotor is rotatable about said axis into a position in which the second port is at least substantially obstructed and either the third port or the first port is at least partially obstructed.
 7. A turbocharger system according to claim 6, wherein the valve rotor is rotatable about said axis into one of a plurality of positions in which the second port is at least partially obstructed and either the third port or the first port is at least partially obstructed.
 8. A turbocharger according to claim 6, wherein the second port is fully obstructed.
 9. A turbocharger according to claim 6, wherein the first port is unobstructed and the third port is at least partially obstructed.
 10. A turbocharger system according to claim 2, wherein the valve rotor is rotatable into a position in which the first port is at least substantially obstructed and either the third port or the second port is at least partially obstructed.
 11. A turbocharger system according to claim 10, wherein the valve rotor is rotatable about said axis into one of a plurality of positions in which the first port is at least substantially obstructed and either the third port or the second port is at least partially obstructed.
 12. A turbocharger system according to claim 10, wherein the first port is fully obstructed.
 13. A turbocharger according to claim 10, wherein the second port is unobstructed and the third port is at least partially obstructed.
 14. A turbocharger system according to claim 13, wherein the valve rotor is rotatable into a position in which the third port is fully obstructed.
 15. A turbocharger system according to claim 1, wherein the junction is downstream of said first turbine.
 16. A turbocharger system according to claim 2, wherein the junction is downstream of the first turbine, the first port is an inlet port communicating with the first turbine, the second port is an inlet port communicating with the bypass exhaust gas passage, and the third port is an outlet port communicating with the second turbine.
 17. A turbocharger system according to claim 1 wherein the junction is upstream of the first turbine.
 18. A turbocharger system according to claim 1 wherein the junction is upstream of the first turbine, the first port is an inlet port communicating with the first exhaust gas passage, the second port is an outlet port communicating with the bypass exhaust gas passage, and the third port is an outlet port communicating with the second turbine via the first turbine.
 19. A turbocharger system according to claim 18, wherein the valve rotor is rotatable into at least one position in which the second port is at least substantially obstructed and the first port is at least partially obstructed.
 20. A turbocharger system according to claim 19, wherein the second port is fully obstructed and the third port is unobstructed.
 21. A turbocharger system according to claim 18 wherein the valve rotor is movable into at least one position in which the third port is at least substantially obstructed and the first port is at least partially obstructed.
 22. A turbocharger system according to claim 21, wherein the valve rotor is rotatable into a position in which the third port is fully obstructed and the second port is unobstructed.
 23. A turbocharger system according to claim 1, further comprising a pressure release valve for venting gas from either the exhaust bypass gas passage or the first exhaust gas passage upstream of the second turbine inlet.
 24. The turbocharger system according to claim 23, wherein the pressure release valve is operable to maintain pressure within the exhaust gas bypass passage and/or first exhaust gas passage within a desired range and/or below a desired maximum and/or at a desired value.
 25. A turbocharger system according to claim 23, wherein the pressure release valve is biased into a closed position and operates to open when pressure within the bypass gas flow passage or first exhaust gas passage increases above a threshold sufficient to move the valve into an open position against the biasing force.
 26. A turbocharger system according to claim 1, wherein the rotary valve is housed within a housing which is separate to or integral with either one of the first and second turbine housings.
 27. A turbocharger system according to claim 1, wherein the first and second turbines have a common turbine housing, the rotary valve being housed within said common turbine housing either upstream or downstream of the first turbine inlet.
 28. A turbocharger system according to claim 2, wherein the valve rotor comprises an arcuate surface which at least partially covers the first and/or second, and/or third port when the valve rotor is in a position in which the respective port is at least partially obstructed.
 29. A turbocharger system according to claim 28, wherein said arcuate surface defines a sector of a cylinder.
 30. A turbocharger system according to claim 1, wherein the valve rotor is rotatably supported at first and second ends thereof for rotation about said axis.
 31. A turbocharger system according to claim 1, comprising an electric, pneumatic, or hydraulic actuator for effecting rotation of the valve rotor.
 32. A turbocharger system according to claim 1, wherein the rotary valve is adapted to receive control signals from a controller such as an engine control unit (ECU).
 33. A turbocharger system according to claim 1, further including at least one sensor from the following group: a sensor for detecting or determining the speed of rotation of the first turbine and/or the second turbine, a sensor for detecting or determining the boost pressure generated by the first and/or second turbocharger; a sensor for detecting or determining the back pressure in the exhaust gas bypass path; a sensor for detecting or determining pressure within the rotary valve chamber; a sensor for detecting or determining the rotational speed of an engine to which the turbocharger system is operably connected; and a sensor for detecting or determining load on an engine to which the turbocharger system is operably connected.
 34. A turbocharger system according to claim 1, further comprising a second turbocharger exhaust bypass flow passage communicating between the outlet of the second turbocharger and the inlet of the second turbocharger downstream of said junction, and a bypass flow control valve for selectively permitting exhaust gas flow to bypass said second turbine.
 35. A turbocharged internal combustion engine including a turbocharger system comprising: a first relatively small turbocharger; a second relatively large turbocharger; the first turbocharger comprising a first exhaust turbine situated in a first exhaust passage; the second turbocharger including a second exhaust turbine situated in said first exhaust passage downstream of said first turbine; an exhaust bypass flow passage communicating with the first exhaust flow passage upstream and downstream of the first turbine; and a rotary valve located at a junction of the bypass flow passage and the first exhaust gas flow passage, the rotary valve comprising a valve rotor which is rotatable about a valve axis to selectively permit or block flow to the second turbine inlet from the first exhaust gas passage and/or bypass gas passage.
 36. A turbocharged internal combustion engine according to claim 35, including an exhaust gas re-circulation system comprising a re-circulation gas flow path from an exhaust side to an air intake side of the engine, and an exhaust gas re-circulation valve provided in said path for controlling re-circulation of exhaust gas to the air intake side of the engine, wherein said rotary valve is operable to modulate back pressure in the exhaust gas flow to assist in the exhaust re-circulation.
 37. A turbocharger, comprising a turbine wheel mounted for rotation within a turbine chamber defined by a turbine housing, the housing defining a first inlet passage, a second inlet passage, and a third passage communicating with said turbine chamber, the first, second and third passages meeting at a junction, wherein a rotary valve is located at said junction, the rotary valve comprising a valve rotor which is rotatable about a valve axis to selectively permit or block exhaust gas flow to the third inlet passage from either the first inlet passage and/or second inlet passage.
 38. A turbocharger according to claim 37, wherein the valve rotor rotates within a valve chamber defined by the housing, the valve chamber having a first inlet port communicating with the first inlet passage, a second inlet port communicating with the second inlet passage, and an outlet port communicating with the third inlet passage.
 39. A turbocharger according to claim 38, wherein the valve rotor is rotatable about said axis into a position in which each of the three ports is at least substantially unobstructed.
 40. A turbocharger according to claim 38, wherein the valve rotor is rotatable about said axis into at least one position in which the first inlet port and the outlet port are unobstructed, and the second inlet port is at either partially or fully obstructed.
 41. A turbocharger according to claim 38, wherein the valve rotor is rotatable about said axis into at least one position in which the first inlet port is unobstructed, the second inlet port is fully obstructed and the outlet port is either partially or fully obstructed.
 42. A turbocharger according to claim 38, wherein the valve rotor is rotatable into at least one position in which the second inlet port is unobstructed, the first inlet port is fully obstructed, and the outlet port is partially or fully obstructed.
 43. A turbocharger according to claim 38 wherein the valve rotor is rotatable into a position in which the outlet port is fully obstructed.
 44. A turbocharger according to claim 37 further comprising a pressure release valve for controlling or limiting pressure within the second inlet passage and/or the valve chamber. 